The fact that protests in Libya continue despite the killing of at least four demonstrators since Tuesday suggests that Muammar Gaddafi could be facing the most sustained challenge yet to his 41-year rule. But there are crucial differences between Libya and its neighbors, Tunisia and Egypt, where the overthrow of dictators since the New Year has passed largely peacefully, and Gaddafi's regime is unlikely to fall without a bloody fight.

Armed pro-government demonstrators clashed with Gaddafi opponents on what opposition groups had dubbed a "Day of Rage", Thursday, and the AP has reported dissidents claiming that at least 14 people have been killed in the clashes. The toll may be even higher. Unverifiable claims on Twitter, for example, include on resident of the eastern city of al-Baida claiming that 35 people were killed there on Thursday. Earlier in the day, hospital staff had reported shortages of medical supplies as casualties mounted.

A near total news blackout and the regime preventing foreign journalists from entering the country has meant there's little independent  or verifiable  coverage of Libya's biggest protest in years. But residents have uploaded cellphone videos to Facebook pages, tweeted to foreign-based Twitter accounts, and phoned information in to Al Jazeera and other Arab satellite channels. Mobile phone service to Benghazi and al-Baida was severely disrupted on Thursday.

The Libyan protestors' grievances closely track those in Tunisia and Egypt: Soaring youth unemployment, and the harsh suppression of all political activity under an autocratic rule which has lasted decades in the country of 6.5 million. The key difference, however, could be Libya's military, whose commanders are aligned with Gaddafi, who had been a 27-year-old colonel when he seized power in a 1969 coup. While Tunisian and Egyptian commanders refused to open fire on protesters or use force to end demonstrations, Libyan protesters are unlikely to find such sympathy from their country's soldiers. Instead, Libya's security police have shown their readiness to fire live ammunition, according to human-rights and exile groups, who cite videos posted to YouTube from this week's protests in the port city of Benghazi and smaller cities; that footage shows bursts of gunfire, as demonstrators flee a police assault.

"The violence we saw in Egypt will probably be a fraction of what we could see in Libya," says Ahmed Addarrat, 24, a Libyan-American in Orlando, Florida, who is active in an anti-government organization called Enough/Khalas, comprised largely of second-generation Libyans, which began after Gaddafi's defiant speech to the U.N. General Assembly in New York in 2009. Addarrat says he has received numerous video uploads since Tuesday from Libyan protesters, who say they fear fierce attacks. "This is not a case where the military will shift with the people," he says.

The simple act of protesting peacefully, under those circumstances, is to risk one's life  and the fact that thousands of Libyans have been willing to take that risk over the past week underscores the depth of feeling against Gaddafi. Large crowds demonstrated on Tuesday in the Mediterranean port city of Benghazi, which has long been a stronghold of anti-government sentiment. Those crowds joined a far smaller demonstration already underway, called by some of the families of 1,200 inmates killed in a 1996 massacre in the Abu Salim prison. The resulting clashes sparked more protests in the cities of Baida and Zentan, as well as pro-Gaddafi demonstrations in Benghazi, Tripoli and elsewhere. Thursday's call for a nationwide strike and mass demonstrations could signal whether the protests can maintain their momentum in the face of the regime's willingness to unleash violence. "How serious a challenge this is to Gaddafi will depend on how many people will take the risk of going out on the street," says Heba Morayef, researcher for Human Rights Watch in Cairo. She points out that the 1996 prison massacre, in which police gunned down about 1,200 inmates, was in response to the last serious attempt to overthrow Gaddafi, which also originated in Benghazi. "That response remains very live in people's memory," Morayef says.

But the collapse of the regimes of the Ben Ali regime in Tunisia and the ouster of Egypt's President Hosni Mubarak has drastically changed the calculus for Gaddafi retaining power, according to many Libya watchers. "After Tunisia and Egypt everything is possible," says Guma el-Gamaty, a Libyan anti-government writer and business studies professor at Grafton College in London. "The books have been rewritten." Similarly, Morayef says Libyans have told her in recent days that Mubarak's ouster in particular was a signal that Gaddafi might be equally vulnerable in the face of a determined popular revolt. "Egypt is also seen as a big, heavily military-controlled dictatorship," Morayef says. "The Libyans I've spoken to say, 'if Egypt can do it, we should be able to do it too.' "